The present invention relates to injection molding and more particularly to a stack mold configuration utilizing a movable sprue bar.
Stack molding provides the advantage that it enables an injection molding machine output to be doubled without significantly increasing its size. Stack mold configurations generally employ a stationary first platen, a movable center platen and a movable second platen. The cavities are conventionally located on opposing faces of the movable center platen. Advantageously, the same machine forces are applied to two cavities in series rather than to a single cavity. However, stack molding has the disadvantage that the melt runner system must extend through the moving center platen in order to reach the cavities.
Conventionally, multi cavity stack molds use a sprue bar which is a direct channel between the machine's extruder nozzle and the molds hot runner distributor mounted in the center section of the stack mold. In the case of large parts, one part being mounted on either side of the stack molds center platen, the part nearest the machine's extruder prevents the use of a conventional sprue bar. Therefore, an alternate means of conveying the resin to the center section must be devised.
U.S. Pat. No. 4,539,171 to Sorensen utilizes the a snorkel device which is essentially a sprue bar offset from and parallel to the central axis of the clamping platens. The snorkel of the '171 patent is attached to the stack molds hot runner and makes a seal with the injection molding machine's extruder nozzle by the action of the clamp closing the mold. This design is characterized by several disadvantages. Firstly, the machine's nozzle must be specially modified to extend its channel to the offset position chosen for the snorkel. In addition, a hole must be made in the stationary platen through which the snorkel and the extended nozzle may pass. Therefore, the design of the '171 patent requires expensive machine modifications.
There is an additional disadvantage of the design of the '171 patent that the action of sealing the sprue bar against the machine nozzle changes the clamping force applied to the half of the stack mold next to the extruder. This is a disadvantage shared by all conventional centrally mounted sprue bars. Thus, for example, the force required to maintain the nozzle to sprue bar seal is typically 20 tons and is usually applied by a hydraulic cylinder means acting between the extruder assembly and the stationary platen. This force is directly opposing the main clamping force applied between the machine's two platens attempting to keep the stack mold closed during injection. This main clamping force, for example, is typically 500 tons. Because the 20 ton force required to maintain the nozzle to sprue bar seal is transmitted directly to the center section of the stack mold it effectively subtracts 20 tons from the clamping force acting on the mold set next to the extruder and adds 20 tons to the mold set next to the moving platen. Thus, the two halves of the stack mold see 480 tons and 520 tons respectively. Consequently, disadvantageously, the filling of the two halves is not symmetrical.
U.S. Pat. No. 4,212,626 and 4,586,887, both to Gellert show a means to convey the resin to the parts by eliminating the sprue bar and making the channel seal at the molds parting line nearest the extruder. In the '626 patent the resin is fed through a first part 76 to a second part 78. Closing the resin channels to prevent disadvantageous drooling is achieved by means of valve gates at each surface. Hence, three valve gate nozzles are used. In the '887 patent a conventional multi-cavity stack mold is shown where the sprue bar is replaced with a nozzle. The nozzle seals to a hot runner manifold bushing at the parting line of the part nearest the machine's extruder. The channels are not closed at the parting line after the resin has been conveyed, instead a valve gate is used to close the hot runner manifold inlet channel. The design of both the '626 and '887 patent have the disadvantage of allowing drool to escape on the parting line surface. This is a considerable disadvantage. No matter how efficient the valve gates are in the '626 patent, or the fact that machine suckback may be employed to decompress the molds hot runner manifold, in both patents a small amount of drool will in fact escape. This cannot be tolerated at the parting line since it prevents complete mold closing and allows flashing to occur. Continued operation of the mold with slight drool causes permanent damage requiring expensive repairs.
An additional method is shown in Kunstoffe, June 1988 "Development and Construction of a Stack Mold" Pages 505-510, By R. Strauch. This design utilizes a sprue bar displaced from the mold center line and clamped at both ends to part of the molds hot runner system. The machine nozzle seals to a bush conventionally and resin is conveyed via a first manifold through a permanent seal to the sprue bar, and through a second permanent seal to the second manifold in the stack mold center section. This design has the advantage of being able to be installed in an unmodified machine, however, it suffers from the disadvantage that the ejector stroke and opening stroke for the part nearest the machine's extruders is severely limited. The ejector mechanism is incorporated within the mold structure on the core half of the mold. In accordance with this design, the strokes must be built into the shut height of the mold, and this severely limits the height of the parts that can be molded in this way in an unmodified machine.
Accordingly, it is a principal object of the present invention to provide an improved injection molding method and apparatus.
It is a still further feature of the present invention to provide an improved injection molding method and apparatus a aforesaid employing a stack mold configuration and utilizing a movable sprue bar.
It is a still further object of the present invention to provide an improved method and apparatus as aforesaid which is inexpensive and convenient to build and operate and which overcomes the disadvantage of differential clamping force applied to each half of a stack mold.
Further objects and advantages of the present invention are appeared hereinbelow.